Process for preparing esters



United States Patent 3,546,274 PROCESS FOR PREPARING ESTERS Walter L.Borkowski, Media, and William D. Vanderwerif, West Chester, Pa.,assignors to Sun Oil Company, Philadelphia, Pa., a corporation of NewJersey No Drawing. Continuation-impart of application Ser. No. 337,547,Jan. 14, 1964. This application Jan. 27, 1967, Ser. No. 612,096

Int. Cl. C07c 69/76 US. Cl. 260-475 8 Claims ABSTRACT OF THE DISCLOSUREImprovements in the preparing of dialkyl esters of naphthalenecarboxylic acids from dialkyl naphthalenes in which separation of theintermediate carboxylic acid and the subsequent esters from by-productsis facilitated in the respective stages by using at least some lithiumchromate as the alkali metal chromate oxidizing agent and carrying outboth the oxidation and the esterification under a C0 atmosphere. Theseparation is facilitated because of the formation of water solublecarboxylic acid salts and water insoluble inorganic salts as by-productsin the oxidation step and the formation of alcohol soluble esters andalcohol insoluble by-product salts in the esterification step of theprocess. The dialkyl naphthalenes can be obtained as a gas oil extract.The oxidation is carried out at about 100 to 300 C. The esterificationis carried out with an alcohol of about 1 to 12 carbon atoms and atabout 50 to 200 C.

This application is continuation-in-part of Ser. No. 337,547 filed onJan. 14, 1964, now US. Pat. 3,330,862, by the present inventors based onmatter divided out of that application.

This invention relates to a novel process for the preparation ofnaphthalenedicarboxylic esters, and more particularly an an improvedprocess for the production of 2,6-naphthalenedicarboxylic esters by theoxidation of 2,6-dialkylnaphthalene.

Naphthalenedicarboxylic acids and their lower alkyl esters, particularlydimethyl 2,6-naphthalenedicarboxylate, are useful in the preparation ofpolyester fibers and films. The monomer acids desired as such or as anintermediate to the esters may be prepared by direct oxidation of eithersubstantially pure 2,6-dimethylnaphthalene, or the more readilyobtainable mixtures of alkylnaphthalene isomers such asmethylnaphthalenes, dimethylnaphthalenes, trimethylnaphthalenes,ethylnaphthalenes and the like, such as those found in catalytic gas oilfractions obtained by solvent extraction of a catalytic gas oildistillation fraction boiling in the range of from about 485 to 515 F.to form the corresponding carboxylic acids. Thus, for example, onemethod of preparing naphthalenedicarboxylic acids (sometimes referred toas NDCA) from dialkylnaphthalenes has been to oxidize adimethylnaphthalene (sometimes referred to as DMN) with sodiumdichromate in accordance with the following equation:

While this is generally a satisfactory method, it is nec essary toemploy a buffered reaction system since the NaOI-I produced during theoxidation will convert the remaining dichromate to unreactive chromate:

Na2CI'207+2NaOH'- 2Na CrO -i-H O In practice, a convenient buffer is a50% excess of the dichromate oxidizing agent; the reaction then becomes:

'ice

This excess of dichromate is not only costly, but it thereafter makesthe resulting separation of the product and spent reagent, as well asthe recovery of the sodium chromate, technically and economicallyunattractive.

Therefore, it is an object of the present invention to provide a methodof oxidizing alkylnaphthalenes, and particularly dialkylnaphthalenessuch as 2,6-dimethylnaphthalenes to the correspondingnaphthalenecarboxylic acids and particularly the esterification thereofwith substantial economy and simplicity of operation over priorprocesses.

It has now been found, in accordance with the present invention, thatthese and other objects may be achieved by oxidizing alkylnaphthaleneswith a suitable oxidizing agent in the presence of carbon dioxide, thecation of said oxidizing agent having the property of forming awater-soluble salt with the naphthalenecarboxylic acid but not with theresulting carbonic acid, followed by the esterification of said acidwith an alcohol of from about 1 to 12 carbon atoms, in the presence ofcarbon dioxide.

This process is most advantageous in that the carbon dioxide not onlyfunctions as the buffering agent, but in addition, it precipitates theselected cation as the insoluble carbonate; thus the salts of theproduct naphthalenecarboxylic acids are the only water-soluble speciespresent at the completion of the oxidation reaction and their isolationrequires only filtration and evaporation of the water and in certainmarkedly dilferent preferred embodiments in the preparation of theesters, the solubility-insolubility (in alcohol) of product andby-product are also used to facilitate recovery of the ester. While thismethod is preferably employed in the oxidation of a singlealkylnaphthalene, nevertheless, it is entirely suitable in the case ofmixed isomers for the preparation of mixed carboxylate salts followed byesterification whereby a mixture of esters obtains.

The cations of the oxidizing agents useful in this process forminsoluble carbonates, yet they can be readily recovered and regeneratedto the corresponding chromate by known methods for further reuse in theoxidation process.

In one embodiment of this process, the alkylnaphthalene startingmaterial may be oxidized to form the corresponding carboxylate salt inaccordance with the following equation:

This reaction is desirably carried out in an aqueous medium at atemperature of about -300 C. and preferably at about 250 C., for about2-6 hours, at a pH of from about 4 to 8, the carbon dioxide having beenintroduced under pressure into the reaction vessel before the reactionis initiated. The reaction mixture is then cooled and filtered to removethe precipitated Cr O and Li CO the filtrate is extracted with asuitable organic solvent such as ether, or a low boiling hydrocarbonsolvent to remove any unreacted starting material. The remainingfiltrate is then boiled for an additional 1-3 hours to convert anysoluble LiHCO to Li CO which is then removed by filtration.

The lithium salt may then be directly esterified with an alcohol ofabout 1 to 12 carbon atoms, as for example with methanol, ethanol,butanol, hexanol, nonanol, and dodecanol, to form the correspondingdialkylnaphtha lenedicarboxylate. This esterification can be carried outin accordance with the following illustrative equation wherein thepreferred alcohol, methanol, is used:

This reaction, which is characterized by the recovery of additionallithium carbonate suitable for regeneration to lithium chromate, iscarried out at a lower temperature than the original oxidation on theorder of 50200 C. preferably at about IOU-150 C., the CO in the specificcase of the lithium carboxylate being introduced at a pressure of from100 to 1000 p.s.i. As in the case of the first step of oxidizing thehydrocarbon to the carboxylate, the reaction is not pressure dependent;however, pressure is needed to contain the required amount of CO as wellas the alcohol at the high temperatures employed.

In a further embodiment of this process, it has been found that in anatmosphere of carbon dioxide, if a combination of oxidizing agents isemployed in which one of the cations is lithium and the other cation iseither sodium, potassium, cesium, or a mixture thereof, the latter namedcations will react with the carboxylic acid formed by the oxidation toform the desired Watersoluble carboxylate salt and this in turn willresult in facilitated separation. Thus, for example, potassium chromate,sodium chromate, cesium chromate or the like can be utilized incombination with lithium chromate in accordance with the followingequation:

where M is potassium, sodium or cesium. The resulting2,6-naphthalenedicarboxylate salt, or mixture of isomericnaphthalenedicarboxylate salts, depending on the aromatic charge, maythen be esterified. In this case the ester is soluble in the alcoholmedium used in the estrification but the potassium, sodium or cesiumcarbonate product in this case, like the lithium carbonate in the firststep, is insoluble so that the ester is easily separated and recovered.Of course, it can now be appreciated that the water formed in theesterification is removed as formed to maintain the reaction relativelyanhydrous. A clear excess of CO over that required to form the carbonateis required to maintain an acidic medium. The reaction conditionsemployed when combinations of oxidizing agents are used are essentiallyas described hereinabove with respect to the first embodiment. Thusfollowing the introduction of carbon dioxide and heating the reactionmixture for from 2 to 6 hours, the precipitated oxides and carbonatesare removed by filtration and the filtrate extracted with an ether toremove unreacted starting material. In the case where crude aromaticcharge stocks are used as starting materials, other substances such asdecalins, tetrahydronaphthalenes, acenaphthenes and the like which maybe present are converted to the corresponding tetralones, quinones, etc.and are likewise removed by the ether extraction step. If the initialreaction mixture is heated sufficiently prior to filtering off theprecipitates, additional heating to convert the intermediate LiHCO isnot necessary, in which case the reaction product may readily berecovered by evaporating the filtrate to dryness.

It will be recognized by those skilled in the art that the amounts ofthe reactants and reaction conditions will be varied in accordance withthe nature of the starting material, depending upon the relativeproportions of mono-, diand trialkylnaphthalenes in the charge stock aswell as non-naphthalenic materials. It will be appreciated from theforegoing description that one of the unique advantages of this modifieddialkylnaphthalene oxidation and esterification process for thepreparation of naphthalenedicarboxylic esters is that the precipitatedcarbonate may be regenerated conveniently, and in some cases at leastwithout prior separation or purification, to reform the originaloxidizing agent suitable for reuse. Thus, for example, when the onlycation employed is lithium, the resulting mixture of lithium carbonateand chromic oxide may be regenerated by kiln roasting, pressureoxidation or other well-known means to form lithium chromate. Similarly,where cesium, sodium, or potassium dichromate or chromate is employed,the cation which is removed in the form of the carboxylate salt isreadily replenished by the addition of the corresponding hydroxide,carbonate or other suitable form to the precipitated solids prior toregeneration, and the lithium, cesium, sodium and/or potassium chromatemixture or the like is recovered. The use of lithium chromate alone isnot preferred in the first step because, although it facilitatesseparation of. the intermediate carboxylic salt due to the insolubilityof the M 00 it is more expensive than the corresponding sodium andpotassium compounds.

The esterification is conveniently represented and easily understoodfrom the following equation:

where M, as in Equation 6, is sodium, potassium or cesium.

The following are examples given by way of illustration and are not tobe regarded as limitations of this invention:

EXAMPLE 1 28.4 g. of a catalytic gas oil extract containing 20% byweight of methylnaphthalenes, 8% ethylnaphthalenes, 53%dimethylnaphthalenes, 9% trimethylnaphthalenes and 10% other aromaticsis charged to a one-liter rocking autoclave with 29.3 g. (0.15 mole) ofpotassium chromate, 58.5 g. (0.45 mole) of lithium chromate and 200 ml.of water. The autoclave is then pressured with 480 p.s.i. of carbondioxide and subsequently heated at 250 C. for four hours. The crudereaction mixture is filtered, the solid washed with ether and thefiltrate extracted with ether. Evaporation of the ether washes andextracts gives 8.4 g. of unreacted hydrocarbons. The solids consist of49.5 g. of chromic oxide and 10.9 g. of lithium carbonate. The aqueousfiltrate is boiled for two hours, then filtered hot to obtain 17.4 g. ofprecipitated lithium carbonate. Evaporation of the filtrate gives 31.4g. of potassium salts of mixed naphthalenecarboxylic acids (98.5% oftheory) and 5.4 g. of lithium carbonate.

The recovered lithium carbonate is combined with the filter cake fromthe first filtration, which consists of a mixture of chromic oxide andlithium carbonate, for regeneration of the lithium chromate potassiumchromate mixture by roasting with added potassium carbonate.

EXAMPLE 2 27.8 g. of catalytic gas oil extract is charged to a oneliterrocking autoclave with 58.5 g. of lithium chromate (0.45 mole), 57.3 g.of cesium chromate (0.15 mole), and 200 ml. of water. The autoclave isthen pressured with 480 p.s.i. of carbon dioxide and then heated at 250C. for four hours. The crude reaction mixture is boiled vigorously fortwo hours and filtered hot, the solids washed with ether and thefiltrate extracted with ether. The solids consist of 49.5 g. of chromicoxide and 17.4 g. of lithium carbonate. The ether washes and extractscontain 8.3 g. of unreacted hydrocarbons. Evaporation of the aqueousfiltrate gives a 98% yield of cesium salts of mixednaphthalenecarboxylic acids containing 4.0 g. of lithium carbonate.

The recovered lithium carbonate is combined with the filter cake fromthe first filtration, which consists of a mixture of chromic oxide andlithium carbonate, for regeneration of the lithium chromate cesiumchromate mixture by roasting with added cesium carbonate.

EXAMPLE 3 To a one-liter rocking autoclave is charged 27.5 g. of anaromatic extract of catalytic gas oil containing approxi mately 90% byweight of a mixture of methyl-, ethyl-, dimethylandtn'methyl-naphthalenes, and 10% other aromatics, 58.5 g. (0.45 mole) oflithium chromate, 200 ml. of water and 480 p.s.i. of carbon dioxide. Theautoclave is then heated at 250 C. for four hours, cooled and thereaction mixture filtered. The filter cake and filtrate are both washedwith ether. Evaporation of the latter gives 7.5 g. of unreactedhydrocarbons. The aqueous filtrate is boiled vigorously to convert thelithium bicarbonate to insoluble lithium carbonate which is thenfiltered off hot. The aqueous filtrate is dried to yield 24.9 g. of thelithium salts of the mixed naphthalenecarboxylic acids.

Acidification with hydrochloric acid of the above aqueous filtrate oflithium salts, in lieu of drying the filtrate yields 21.5 g. of mixednaphthalenecarboxylic acids; an additional 2.2 g. of water-soluble acidsis recovered from solution by extraction with ether. The total yield ofacids is 98.5% of theory.

EXAMPLE 4 31.2 g. (0.20 mole) of 2,6-dimethylnaphthalene is charged to aone-liter rocking autoclave with 24.3 g. (0.15 mole) of sodium chromate,58.4 g. (0.45 mole) of lithium chromate, 200 ml. of water and 480 p.s.i.of carbon dioxide and heated for four hours at 250 C. The crude reactionmixture is filtered and the solids washed with ether and the filtrateextracted with ether. The filtrate is then boiled vigorously for twohours and filtered hot to remove the precipitated lithium carbonate.Evaporation of the aqueous filtrate yields 69.8 g. of the disodium saltof 2,6-naphtha1enedicarboxylic acid (97% yield based ondimethylnaphthalene reacted).

EXAMPLE 5 To a one-liter rocking autoclave is charged 31.2 gms. 0.20mole) of 2,6-dimethylnaphthalene, 103.9 gms. (0.80 mole) of lithiumchromate, 200 ml. of water, and 480 p.s.i. of carbon dioxide. Theautoclave is heated at 250 C. for three hours. The cooled reactionmixture is then filtered and both the filter cake and filtrate arewashed with hexane. Evaporation of the washes yields 0.8 gm. unreacted2,6-dimethylnaphthalene. The aqueous filtrate is boiled vigorously forthree hours and filtered hot to recover the pricipitated lithiumcarbonate. The filtrate from the second filtration is evaporated toyield 43.2 gms. of the lithium salt of 2,6-naphthalenedicarboxylic acid.

EXAMPLE 8 32.4 gms. of the potassium salts of mixednaphthalenecarboxylic acids prepared according to Example 1 are chargedto a one-liter rocking autoclave with 150 ml. of methanol. The autoclaveis then pressured with about 400 p.s.i. of carbon dioxide andsubsequently heated at about 125 C. for three hours. The crude reactionmixture is filtered, and evaporation of the methanol gives a good yieldof dimethyl naphthalenedicarboxylate in a high state of purity.

We claim:

1. A process for the preparation of dialkyl esters of naphthalenedicarboxylic acids which comprises oxidizing catalytic gas oil extractcontaining a mixture of isomeric alkyl naphthalenes with an alkali metalchromate oxidizing agent consisting essentially of a combination oflithium chromate and a member selected from the group consisting ofsodium chromate, potassium chromate, cesium chromate, and mixturesthereof, in the presence of carbon dioxide at a temperature of about to300 C. to form an alkali metal naphthalene-carboxylate salt, esterifyingsaid alkali metal naphthalene-carboxylate salt with an alkanolcontaining from 1 to about 12 carbon atoms at from 50 C. to 200 C. inthe presence of carbon dioxide and recovering a naphthalene-carboxylateester.

2. A process according to claim 1 wherein the esterification temperatureis in the range of about 100-150 C.

3. A process according to claim 1 wherein the carbon dioxide pressure isin the range of about 100 to 1000 p.s.i.

4. A process according to claim 1 wherein the esterification temperatureis in the range of about l00150 C., the carbon dioxide pressure is inthe range of about 100 to 1000 p.s.i. and the second said chromateoxidizing agent is sodium chromate.

5. A process according to claim 1 wherein the esterification temperatureis in the range of about 100-150 C. the carbon dioxide pressure is inthe range of about 100 to 1000 p.s.i. and the second said chromateoxidizing agent is potassium chromate.

6. A process according to claim 4 wherein the catalytic gas oil extractis 2,6-dimethylnaphthalene.

7. A process according to claim 5 wherein the catalytic gas oil extractis 2,6-dimethylnaphthalene.

8. A process according to claim 1 wherein the lithium chromate isemployed in a mole ratio to said other metal chromate of about 3:1 ofsaid other metal chromates.

References Cited UNITED STATES PATENTS 2,005,774 6/1935 Demant 260--524LORRAINE A. WEINBERGER, Primary Examiner E. J. SKELLY, AssistantExaminer U.S. Cl. X.R. 260-524

